Hereinafter, general modulation technology used in data communication will be described in brief.
Modulation means that signal information, such as intensity of signal (information), displacement, frequency and phase, is converted into a proper type suitable for channel features of a transmission medium. A transmitter can transmit signal information with a long distance by modulating the signal information using a modulation mode suitable for a transmission medium.
When the transmitter modulates data, since the transmitter can perform modulation over a broad frequency band, the transmitter can configure various channels. Also, the transmitter can shorten an antenna length by enhancing frequency. Also, the transmitter can fulfill design requirements such as filtering or amplification during data modulation. Namely, modulation means that a signal containing data is converted into a waveform suitable for a transmission channel.
Phase shift keying (PSK) modulation means that carrier phase is converted in accordance with digital signal information. Since PSK waveform has given amplitude (envelope), it is less affected by level variation of a transmission path and has excellent immunity against symbol error. Also, since carrier is a double side band signal, it has both timing information and frequency information. Accordingly, modulation and demodulation circuits are relatively simple.
Binary PSK is a transmission mode that transmission signals of two values (for example, 0 or 1) to be transmitted correspond to synchronous phase and out-of-phase. Quadrature phase shift keying (QPSK) modulation is a transmission mode that digital signals of two values, i.e., 2 bits of ‘0’ and ‘1’ correspond to four phases of carrier.
Quadrature amplitude modulation (QAM) is mainly used for digital modulation of high speed and performs modulation by simultaneously combining amplitude and phase of carrier (combination of ASK and PSK) to improve transmission efficiency in a limited frequency band.
QAM is also called multi-level modulation. For example, 16-QAM can transfer a total of 2×8=24=16 kinds of information by respectively determining two amplitude values and eight phase values per one carrier. Namely, QAM is a modulation mode that a unit of phase variation is π/6 and one period is divided into 12, wherein phase angles corresponding to 4 of 12 have two different amplitude values of a modulated signal to express a total of 16 kinds of information. Accordingly, 16 values can be identified per one symbol during QAM, and 4 bits correspond to each symbol.
QAM enables amplitude modulation and phase modulation simultaneously. For example, respective components (i.e., I: synchronous component and Q: quadrature component) of two carriers (sine curve and cosine curve) of which phase difference is 90° and which has the same frequency are modulated in accordance with a amplitude shift keying (ASK) mode and can be transmitted by being synthesized with each other. Meanwhile, although two size levels (for example, +1 and −1) corresponding to I channel and Q channel are provided in case of QPSK, a plurality of size levels (for example, 4, 6, 8, 10, . . . ) can be provided in case of QAM. Also, 16 QAM can be configured by combination of four phases having four size levels or eight phases having two size levels.
However, in general technology, distorted symbol is determined when a symbol signal transmitted through a communication channel is recovered. At this time, a decoder decodes an original signal by determining the symbol using the best method in spite of an error that may occur. In this case, a problem may occur in that packets should again be received for error correction in spite of an error occurring in several bits within one packet.